1. Field of the Invention
The present invention relates to a liquid crystal display element having a display layer including liquid crystal drops or liquid crystal microcapsules, and a manufacturing method thereof.
2. Description of the Related Art
The large quantity of paper consumed mainly in offices has become problematic, owing to both the destruction of forest resources to obtain the raw material for paper pulp as well as environmental pollution arising from the disposal and incineration of refuse. However, consumption of paper meant to be short-lived documentation for temporary viewing of electronic information has tended to increase more and more with the spread of personal computers and the development of information-based society, as with the Internet. It is therefore desirable that a rewritable display medium be developed to replace paper.
In recent years, cholesteric liquid crystal display elements have been notable for such merits as a memory property capable of retaining the display without a power source, a bright display gained from non-use of a polarizing plate, and color display being possible even without use of a color filter.
Cholesteric liquid crystal, in which liquid crystal molecules have a helical structure, causes a selective reflection phenomenon where incident light is divided into right-hand circularly polarized light and left-hand circularly polarized light, and a circularly polarized light component corresponding to the torsional direction of the helix undergoes Bragg reflection, and the rest of the light is transmitted. The central wavelength λ and the reflected wavelength width Δλ of reflected light are denoted as λ=n·p and Δλ=Δn·p respectively where helical pitch is p, average refractive index is n and double refractive index is Δn, and reflected light from a cholesteric liquid crystal layer exhibits vibrant color depending on the helical pitch.
Cholesteric liquid crystal having positive dielectric anisotropy can exist in the following three states: a planar state where the helical axis is perpendicular to a cell surface as shown in FIG. 10A and the above-mentioned selective reflection phenomenon is caused with respect to incident light, a focal conic state where the helical axis is substantially parallel to the cell surface as shown in FIG. 10B and incident light is transmitted while being somewhat subjected to forward scattering, and a homeotropic state where a liquid crystal director turns in the direction of an electric field with a deformed helical structure as shown in FIG. 10C and where incident light is transmitted substantially completely.
Of the three states, the planar state and the focal conic state can bistably exist without voltage. The state of orientation of cholesteric liquid crystal, therefore, is not univocally determined for voltage applied to a liquid crystal layer, the state of orientation of which changes from a planar state to a focal conic state and then to a homeotropic state, in that order, as an applied voltage is increased in a case where the planar state is the initial state, and changes from a focal conic state to a homeotropic state, in that order, as an applied voltage is increased in a case where the focal conic state is the initial state. On the other hand, in a case where voltage applied to a liquid crystal layer is abruptly brought to 0, the planar state and the focal conic state are maintained in these respective states, while the homeotropic state changes into the planar state. The three states can be made to undergo mutual transition by the magnitude of applied pulse voltage.
FIG. 11 shows this electrooptical response. In FIG. 11, the curve A denotes the case where the initial state is the planar state and the curve B denotes the case where the initial state is the focal conic state.
A range denoted as (a) in FIG. 11 shows the planar state or the focal conic state (selective reflection state or transmission state), a range denoted as (b) shows a transition region, a range denoted as (c) shows the focal conic state (transmission state), a range denoted as (d) shows a transition region and a range denoted as (e) shows the homeotropic state, which changes into the planar state (selective reflection state) at a voltage of 0. Vpf,90, Vpf,10, Vfh,10 and Vh,90 signify voltage at which normalized reflectance is 90 or 10 (normalized reflectance of 90 or more is regarded as the selective reflection state and that of 10 or less is regarded as the transmission state) before and after the two transition regions.
A layer for absorbing light of the same wavelength as at least the selective reflection color is disposed on the back of a cholesteric liquid crystal layer, so that a reflection type memory display utilizing the planar state and the focal conic state can be achieved.
A cholesteric liquid crystal display element can have a structure in which liquid crystal is contained in space formed between a pair of display substrates to form a continuous phase, or have a structure such as a PDLC (Polymer Dispersed Liquid Crystal) structure in which drop-like cholesteric liquid crystal is dispersed in a polymeric binder or a PDMLC (Polymer Dispersed Microencapsulated Liquid Crystal) structure in which microencapsulated cholesteric liquid crystal is dispersed in a polymeric binder (for example, refer to Japanese Patent Application Publication (JP-B) No. 7-009512, Japanese Patent Application Laid-Open (JP-A) No. 09-236791 and Japanese Patent No. 3178530 (paragraphs 0159 to 0161)).
The use of the PDLC or PDMLC structure restrains the flowability of liquid crystal, and thus disorder of an image due to bending and pressure decreases, achieving a flexible medium. The direct laminating of multiple cholesteric liquid crystal layers allows realization of color display, and also laminating with a photoconductive layer allows realization of a display element for addressing an image with light signals. In addition, a display layer can be formed with a thick-film printing technique so as to offer the advantage of the manufacturing method thereof being simplified to achieve low cost.
However, a cholesteric liquid crystal display element having a PDLC or PDMLC structure has problems in that selective reflection color in the planar state is low in brightness and color purity so as not to allow a clear color display, and that light transmittance in the focal conic state is poor such that contrast decreases due to a turbid black display in a display element provided with a black light-absorbing layer on the back thereof.
The reason why selective reflection color in the planar state is low in brightness as described above is that, as shown in FIG. 12, an area 32y of disordered orientation occurs in the vicinity of an interface of each liquid crystal drop or liquid crystal microcapsule 32 which has a curved surface such as being in a spherical shape, and an effective selective reflection area 32x reduces in the planar state. This also results in unnecessary scattered light in the focal conic state. An effective means for reducing this defect is to render the diameters of the liquid crystal drops larger and uniform and to decrease the total area of interfaces as much as possible. However, the problem arises of surface irregularities of a display layer becoming large due to the enlarged liquid crystal drops as described below.
As shown in FIGS. 13 and 14, conventional structures including PDLC and PDMLC are manufactured by applying a coating solution for a display layer to a display substrate 10 with a coating device 60 (FIGS. 13A and 14A), evaporating a solvent 35 by heating and decompressing (FIGS. 13B and 14B). Within the coating solution, the liquid crystal drops or the liquid crystal microcapsules 32 are dispersed in an aqueous solution of a polymer serving as a binder. Refer to JP-B No. 7-009512, JP-A No. 09-236791 and Japanese Patent No. 3178530 (paragraphs 0159 to 0161)). FIG. 13 shows an example of polydispersion where the liquid crystal drops or the liquid crystal microcapsules are not uniform in particle diameter, while FIG. 14 shows an example of monodispersion where the particle diameters are uniform.
In the case of FIG. 13, when the concentration of nonvolatile components (non-evaporable components) in a coating film is increased by evaporating the solvent, the flowability of a coating layer decreases, causing a phenomenon called flocculation where multiple liquid crystal drops or liquid crystal microcapsules flow integrally, as shown by 33 in FIG. 13B. Drying progresses in a state where individual dispersed liquid crystal drops cannot freely move, and thus the obtained display layer has a structure in which multiple liquid crystal drops or liquid crystal microcapsules are in an accumulated state, and additionally, a leveling effect on the liquid surface does not sufficiently come to bear, whereby a film is produced which easily obtains large surface irregularities in the liquid crystal drop layer, causing a particular disadvantage which will be mentioned later.
The flocculation also occurs in a case where the liquid crystal drops 32 dispersed as shown in FIG. 14 have uniform and large particle diameters. The larger and more uniform the particle diameters are of the dispersed liquid crystal drops, the greater the tendency is for surface irregularities to become large in a liquid crystal drop layer.
When surface irregularities in the liquid crystal drop layer are large, as shown in FIG. 15, a bonding layer 16 cannot completely cover all of the irregularities and an undesirable air layer 38 occurs between the bonding layer 16 and a display layer 30 at the time that opposite display substrates 10 and 20 are laminated. A desirable voltage cannot be applied to areas including air. The areas thus remain in the planar state which is obtained after coating, do not act and cause unnecessary selective reflection light. Light reflected at the interface between the bonding layer 16 and the air layer 38, or between the air layer 38 and the display layer 30, becomes unnecessary backward scattered light, which particularly causes decrease in light transmittance in the focal conic state and a turbid black display as described above.
In addition, the reason why color purity is low in conventional PDLC and PDMLC structures is as follow. As shown in FIGS. 13 to 15, light which has passed through cholesteric liquid crystal is slightly forward scattered when liquid crystal drops are not orderly arrayed in a monolayer. The forward scattered light enters a second liquid crystal drop layer at a smaller incident angle. A phenomenon where the liquid crystal drops in second or more layers reflect light of a shorter wavelength than the original helical pitch according to Bragg's condition (λ=n·P·cos θ) in addition to the original selective reflection light undesirably occurs in a state of liquid crystal drops being accumulated in the direction of thickness, decreasing the color purity of reflection color observed as a result.
Problems have been described so far with regard to a cholesteric liquid crystal display element. Also with regard to the display element of a PDLC or PDMLC structure using nematic liquid crystal or guest-host liquid crystal, when the liquid crystal drops are not orderly arrayed in a monolayer, there is occasionally a problem of large surface irregularities in a liquid crystal drop layer causing incorporation of air at the time that opposite substrates are laminated, and of a varying abundance ratio of liquid crystal to a polymeric binder in the direction of thickness deteriorating threshold steepness.
JP-A No. 09-90321 describes a method in which liquid crystal microcapsules of a uniform size formed from liquid crystal drops coated with a medium having a constant thickness are formed into a monolayer. This method involves immersing a substrate in an emulsion in which liquid crystal microcapsules are dispersed, and pulling the substrate up from the emulsion at a constant velocity to form the monolayer on the substrate.
This method, however, employs the flow accumulation principle where a substrate is pulled out of an emulsion, and according to the wettability of the substrate surface, particles accumulate where the substrate, solution, and air are in contact due to tension acting between the particles partially immersed in the solution. Therefore, the film-forming rate is low, and the method requires much time. Consequently this method is not suitable for manufacturing a large surface-area device. In addition, a complicated mechanism is required, such as a feedback device for controlling the pull-up rate while observing the state of the coating film. Moreover, a process of applying a polymer solution after forming the monolayer is additionally required to flatten the surface of the coating film.
JP-A No. 2002-270495 describes a leveling method at the time that a coating solution is applied to a semiconductor wafer. With this leveling method, a coating solution applied to a semiconductor wafer is spread by a traveling wave that is generated by a traveling-wave generator (a piezoelectric element). The coating solution is spread within an environment where the atmosphere is pressurized to be equal to or more than the saturation vapor pressure of the coating solution so that no solvent vaporizes and thereby leveling is effectively performed. JP-A No. 2002-270495, however, gives no disclosure that liquid crystal drops or liquid crystal microcapsules, obtained from a coating solution containing particulates such as liquid crystal drops or liquid crystal microcapsules, are arrayed in a monolayer.
Accordingly, the invention has been made in view of the above-mentioned problems and there are needs for a liquid crystal display element that is superior in contrast, by forming a flat film in which liquid crystal drops or liquid crystal microcapsules are densely arrayed in a monolayer, and for a method of simply manufacturing such a liquid crystal display element with a large surface area.